Innovative cold joining technologies based on tube forming

Luis M. Alves, Carlos M.A. Silva, and Paulo A.F. Martins*

IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

Received 19 August 2015 / Accepted 20 August 2015

Abstract – This paper is focused on innovative cold joining technologies for connecting tubes and fixing tubes tosheets. The proposed technologies are based on the utilization of plastic instability waves in thin-walled tubessubjected to axial compression and may be seen as an alternative to conventional joining technologies based onmechanical fixing with fasteners, welding and structural adhesive bonding. Besides allowing connecting dissimilarmaterials and being successfully employed in fixture conditions that are difficult and costly to achieve by meansof conventional joining the new proposed technologies also cope with the growing concerns on the demand, lifecycleand recycling of materials.

Key words: Joining, Cold forming, Tubes, Experimentation, Finite element modelling

1. Introduction

Modern lightweight structures employed in architecture,engineering and building construction make use of tubulartrusses in which tubes are connected to each other or fixedto sheets. Tubular joints are also essential elements in air-conditioning, refrigeration, heat-exchangers, supply lines andpipelines to convey fluids from one location to another.

In case of connecting tubes, there are several well-knowntypes of joints based on commercially available tee fittings,saddle adapters and weld-o-lets for standard geometries andmaterials, such as carbon steel, stainless steel, copper and poly-ethylene, among other thermoplastics (Figures 1a–1c).

The main advantage of connecting tubes by means of com-mercially available tee joints is that it is a cost effective solu-tion whenever the quantity of connections to be performed issignificant. However, the utilization of these types of joints islimited by industry standards and always requires cutting andpreparation of tube ends, welding or brazing and, in most cases,quality inspection of the welds.

Custom based solutions are generally based on the utiliza-tion of on nozzle-weld and spin-formed joints (Figures 1dand 1e). Nozzle-weld joints (Figure 1d) require cutting a holein the main tube, shaping a contoured end in the branch tube tomatch the diameter of the main tube and welding along thecontour. Spin-formed joints (Figure 1e) also requires cuttinga hole in the main tube but the difference is that materialaround that hole is subsequently shaped into a tee fitting wherethe branch tube will be brazed or welded.

In case of fixing tubes to sheets, the most widespreadtechnologies are based on the utilization of fasteners (nutsand bolts or rivets), welding and structural adhesive bond-ing (Figures 1g–1i). The majority of these joints are sim-ple to design and easy to assemble and disassemble but theirrange of application is often limited by various technicalconstraints.

For example, mechanical couplings based on fasteners arelimited by the maximum loading capacity that nuts, bolts andrivets can support safely.

Welding joints suffer from dimensional inaccuracies andheat-affected zones resulting from the heat-cooling cycles,from difficulties of welding dissimilar materials (e.g., joiningsteel or aluminium tubes to aluminium or copper sheets), fromthe production of undesirable fumes and smokes in fabrication,and from expensive and time consuming routes related to theabove mentioned quality inspection procedures.

Structural adhesive bonding requires careful surfacepreparation with tight tolerances and is accomplished by plac-ing a thin film of liquid or semisolid adhesives between thecounterfacing surfaces of the tubes and sheets to be joined.The procedure needs time for the adhesive to cure and theresulting joints may experience decrease in performance overtime under adverse environmental conditions.

From what was mentioned before, it can be concluded thatno matter the application and the differences between thetechnologies that are currently available for connecting tubes(Figures 1a–1e) and fixing tubes to sheets (Figures 1g–1i), theiruniverse of application is always limited by aesthetic, physical,chemical, and mechanical requirements.*e-mail: pmartins@ist.utl.pt

Manufacturing Rev. 2015, 2, 16� L.M. Alves et al., Published by EDP Sciences, 2015DOI: 10.1051/mfreview/2015019

Available online at:http://mfr.edp-open.org

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

OPEN ACCESSRESEARCH ARTICLE

Recent developments in joining by forming that are com-prehensively systematized in the state-of-the-art reviews byMori et al. [1] and Groche et al. [2] allow concluding that plas-tic deformation offers great potential to connect tubes and tofix tubes to sheets. Moreover, joining by forming combinesthe growing demands for high productivity, low fabricationcosts and environmental friendliness with high performanceand material versatility. Table 1 summarizes the main differ-ences between joining by forming and by welding.

This paper is built upon recent developments in joining byforming and presents innovative technologies that make use ofaxisymmetric or asymmetric plastic instability waves in thin-walled tubes subjected to axial compression for connectingtubes and fixing tubes to sheet panels at room temperaturein situations where the axis of the branch tube or sheet isperpendicular or inclined to the axis of the main body tube.In case of connecting tubes by cold forming, the paper willalso be focus on recent developments in the joining of tubesby their ends.

This paper aggregates several new technologies of joiningby forming that were recently developed and published by theauthors [3, 5]. These technologies are not only capable ofconnecting tubes and fixing tubes to sheets made from dissim-ilar materials (e.g. metals and polymers) as they can be

successfully employed in fixture conditions that are difficult andcostly to achieve by means of conventional joining technologies.

Figures 1f and 1j present schematic examples of the appli-cation of the new proposed technologies for cases in which theaxis of the branch tube or sheet is perpendicular to the axis ofthe main body tube that will be comprehensively described inthe presentation.

2. New joining technologies

2.1. Inclined connections

The new proposed technologies for connecting tubes [3]and fixing tubes to sheets [4] are schematically shown inFigure 2. The idea behind the technologies is based on control-ling the development and propagation of plastic instabilitywaves in thin-walled tubes subjected to axial compressionbeyond the bifurcation point of the load-displacement curve.

In case of tube joining (Figure 2a) the left and middle sche-matic drawings show the upper and lower dies that are neededto trigger and propagate inclined, out-of-plane, instabilitywaves between contoured dies at the open and closed positions.The rightmost schematic drawing shows an application of this

Figure 1. Conventional and new proposed technologies for connecting tubes and fixing tubes to sheets.

Table 1. Summary of the main features of joining by forming and by welding.

Joining by forming Joining by welding

Mechanism Plastic deformation Melting with addition of filler materialsShape of the connections Arbitrary geometries Limited to butt, lap, corner and edge jointsOperating temperature Ambient Melting pointHeat-affected zones No YesShielding gases No YesMaterials Metals and polymers Metals (similar)Coated materials Possible Very difficult or impossibleEnergy consumption Less MoreProductivity More LessCost Less MoreEnvironmental friendliness More Less

2 L.M. Alves et al.: Manufacturing Rev. 2015, 2, 16

concept for producing inclined tube joints. The sectional viewsshow the active tool components consisting of the upper andlower contoured dies and the internal mandrels (if present).The internal diameter of the dies is dedicated to a specificreference radius r0 of the main body tube. The radius Rd ofthe parting out-of-plane surface of the dies together with itsinclination a to the axis of the main body tube are dedicatedto a specific instability wave or, in case of tube joining, to aspecific radius of the branch tube. The difference betweenthe radius Rdþ2 t0 and Rd of the upper and lower parting sur-faces is crucial to accommodate the plastic compression beadat the end of stroke. The initial gap opening lgap between theupper and the lower contoured dies controls triggering andpropagation of the plastic instability waves namely, thenumber, width and relative position of the compression beadsalong the axis of the main tube.

Figure 2b shows two different tool systems utilized for fixingtubes to sheets that also make use of axisymmetric (leftmost

setup) or asymmetric (rightmost setup), in-plane, plastic instabil-ity waves. The joints are accomplished by compression beadingand axisymmetric or asymmetric tube end flaring. The plasticinstability waves are produced in appropriate flat or contoureddies whereas flaring is accomplished by compressing the uppertube end with and appropriate radiused punch in order to expandmaterial outwards and to form a single-lap inclined flange.

In connection to what was said above, the main operatingparameters that are considered relevant to the new technologiesfor connecting tubes and fixing tubes to sheets are: (i) the slen-derness ratio lgap/r0 between the initial gap opening and theouter radius of the tube, (ii) the ratio t0/r0 between the wallthickness and the outer radius of the tube, (iii) the mandrel(if present) and (iv) the tribological conditions.

Other relevant operating parameters that are processdependent are: (i) the inclination angle a of the contoured dies,(ii) the radius rc of the flaring punch and (iii) the angle b of thetube end chamfers.

Figure 2. (a) Sectional schematic views of the tool system to develop and propagate inclined, out-of-plane, instability waves in tubes and toproduce inclined tube joints. (b) Sectional schematic views of the tool system to develop and propagate inclined, in-plane, instability waves tofix tubes to sheets.

L.M. Alves et al.: Manufacturing Rev. 2015, 2, 16 3

2.2. End-to-end connections

The new proposed technology for joining two tubes bytheir ends [5] is schematically shown in Figure 3. As seen fromthe open, intermediate and closed positions of the tool system,joining is accomplished by a sequence of three different coldforming stages that are carried out sequentially in a singlestroke: (i) expansion, (ii) local buckling and (iii) clampingby mechanical locking.

Expansion is performed by forcing the upper tube againstthe chamfered end of the lower tube in order to enlarge theunsupported height l0 of the upper tube radially and to createadjacent counterfacing surfaces between the two tubes to bejoined. During this stage the chamfered end of thelower tube acts like a tapered punch (refer to ‘‘first stage’’ inFigure 3).

Once the unsupported height l0 of the upper tube reachesthe end of the depth of insertion lins resulting from the radialclearance between the tube and the upper end of the lowerdie, there is a sudden change in material flow and plastic insta-bility waves are triggered as a result of local bucking underaxial compression loading (refer to ‘‘second stage’’ in Figure 3).The internal mandrel ensures that the plastic instability wavesare formed exclusively outwardly so that design specificationsof the inner diameter of the tube joint are met and alterations inthe flow of liquids or gases are prevented.

Finally, propagation of the plastic instability waves undercontinuous axial compression loading clamps the adjacentcounterfacing surfaces of the two tubes by mechanical locking(refer to ‘‘third stage’’ in Figure 3). It is worth noting thatalthough the new proposed joining process has been developedfor carbon steel tubes it can also be used in tubes made fromother metals or thermoplastics such as polyethylene (PE) andpolypropylene (PP) [6].

The main operating parameters of this technology are sim-ilar to those listed in Section 2.1 but it is worth noticing thatlgap ffi l0 + li + 2lsc in case of joining tubes by their ends andthat the depth of the side clearance lsc that controls the inser-tion of the tubes in the lower and upper dies also plays a rolein the process.

3. Experimentation

3.1. Mechanical characterization of the materials

The experimental development of the new proposed tech-nologies were performed on commercial S460MC (carbonsteel) welded tubes and commercial PVC-U PN10 (unplasti-cized polyvinylchloride). The S460MC tubes were suppliedwith an outer radius r0 = 16 mm and a wall thicknesst0 = 1.5 mm, and were utilized in the ‘‘as-received’’ condition.The PVC-U PN10 tubes were supplied with an outer radiusr0 = 16 mm and a wall thickness t0 =1.6 mm, and weremanufactured in compliance with EN 1452-2 standard for amaximum operating pressure of 10 bar.

The stress-strain curve of the S460MC tubes was deter-mined by combining tensile and stack compression testscarried out at room temperature on a hydraulic testing machine(Instron SATEC 1200 kN). In contrast, the strength differentialeffect of PVC requires determination of the individual stress-strain curves for tension and compression by means of tensileand stack compression tests (Figure 4a).

The tensile test specimens were machined from the sup-plied tube stock in accordance to the ASTM D 638 (PVC-U)and ASTM E8/E8M–09 (S460MC) standards. The stack com-pression test specimens consisted of multi-layer cylinders thatwere assembled by pilling up 3 circular discs with 12 mm

Figure 3. Sectional schematic views of the tool system to connect two tubes by their ends.

4 L.M. Alves et al.: Manufacturing Rev. 2015, 2, 16

diameter and 1.6 mm (PVC) or 1.5 mm (S460MC) thicknesscut out from the supplied tubes by a hole-saw. All the testswere carried out at room temperature on a hydraulic testingmachine (Instron SATEC 1200 kN) with a cross-head speedequal to 10 mm/min.

The critical instability load for the occurrence of localbuckling was determined by compressing PVC and S460MCsteel tubes with 100 mm initial length between flat parallelplatens. As seen in Figure 4b, the shape of the experimentalload-displacement curves that were obtained for the tubularspecimens of each material are similar but the absolute valuesof their critical instability loads Fcr are very different. In fact,the critical instability load of S460MC carbon steel tubes is93.5 kN while that of PVC is 11.3 kN.

3.2. Testing conditions

The experimental setups associated to the different joiningtechnologies that are schematically shown in Figures 2 and 3were installed in the hydraulic testing machine (Instron SATEC1200 kN) where the mechanical characterization of the tubeshad previously been performed. The experiments were carriedout at room temperature with a cross head speed equal to that

utilized in the mechanical characterization of the tube materi-als with the purpose of identifying the modes of deformationand setting up the process feasibility window of the new join-ing technologies as a function of a selected number of operat-ing parameters. The influence of some of these parameters willbe provided later in Section 5.

4. Numerical modelling

Because the experimental development of the new joiningtechnologies for connecting tubes and fixing tubes to sheetswas performed with a quasi-static constant displacement rateof the upper-table of the hydraulic testing machine, no inertialeffects on plastic deformation are likely to occur and thereforeno dynamic effects in the joining process are needed to be con-sidered. These operating conditions allowed numerical model-ling to be performed with the finite element flow formulationand enabled the authors to utilize their in-house computer pro-gram I-form that has been extensively validated against exper-imental measurements of metal forming processes since theend of the 1980’s [7].

I-form is based on the finite element flow formulation andincludes an extension to pressure-sensitive polymers thatallows modelling metal and polymer tubes simultaneously.The extension of the flow formulation to pressure-sensitivepolymers is based on the utilization of a non-associated flowrule in which the plastic potential Q = J2 commonly utilizedin metallic materials is not retrieved from the yield functionF(rij) that explicitly accounts for the strength-differential effectresulting from the differences between tensile rT and compres-sive rc flow stresses [8],

F rij

� �¼ �r 2 � rC � rT þ ðrC � rT Þ rkk ¼ 0; ð1Þ

where �r ¼ffiffiffiffiffiffiffiffiffiffiffiffi32 r0ijr

0ij

qis the effective stress and rkk = dij rij.

The non-associated flow rule where Q = J2 and _k is a sca-lar factor of proportionality is given by,

_epij ¼ _k

oQ rij

� �

orij: ð2Þ

The finite element flow formulation giving support toI-form is built upon the following extended variational state-ment that accounts for contact with friction between rigidand deformable objects:

P ¼Z

V�r _�e dV þ 1

2KZ

V

_e2v dV �

Z

ST

T iui dS

þZ

Sf

Z jur j

0

sf dur

� �dS; ð3Þ

where, _�e is the effective strain rate, _ev is the volumetricstrain rate, K is a large positive constant enforcing theincompressibility constraint of both metals and polymersand V is the control volume limited by the surfaces SU andST, where velocity and traction are prescribed. Friction atthe contact interfaces Sf is treated as a traction boundary con-dition and the additional power consumption term is mod-elled through the utilization of the law of constant frictionsf = mk.

Figure 4. Mechanical characterization of the commercial S460MCand PVC tubes. (a) Stress-strain curves and (b) load-displacementcurves and critical instability loads for the axial compression oftubes between flat parallel platens. The differences in strength ofboth materials justify the existence of two vertical axes in bothplots. S460MC carbon steel refers to the left vertical axis whilePVC refers to the right vertical axis.

L.M. Alves et al.: Manufacturing Rev. 2015, 2, 16 5

The contact between deformable bodies (e.g. between thecounterfacing surfaces of the tubes or between the tubes andsheets, among others) was modelled by means of a non-linearprocedure based on a penalty approach. The procedure is com-prehensively described by Nielsen et al. [9] and has the advantageof avoiding additional degrees of freedom as in case of alterna-tive solutions based on the utilization of Lagrange multipliers.

The tubes and sheets were discretized by means of 2D and3D finite element models as a function of the rotational sym-metry conditions of the joining operation. In case of rotationalsymmetry, the cross section of the tubes and sheets was discret-ized by means of quadrilateral elements across the thickness.In the absence of rotational symmetry, the discretization isfully three-dimensional and performed by means of hexahedralelements. The dies and mandrels were treated as rigid bodies.

5. Results and discussion

5.1. Inclined connections

Figure 5a shows applications of the proposed technologyfor connecting tubes (or half-tubes) in situations where the axisof the branch tube is perpendicular or inclined to the axis of themain body tube. Figure 5b shows applications for fixing tubesto sheets in situations where the axis of the sheet is perpendic-ular or inclined to the axis of the main body tube. The solutionscan, for example, be successfully employed in the seat bottomframe of automotives as an alternative to welding in order toreduce costs and eliminate heat distortion problems. All theconnections shown in Figure 5 were performed betweenwelded carbon S460MC steel tubes, seamless aluminiumAA6062 tubes, aluminium AA5754 sheets and polycarbonatesheets.

Figure 6 shows the results obtained from the numericalmodelling of inclined connections. As seen, Figure 6a presentsthe initial, intermediate and final predicted geometry of a tube

attachment where the axis of the branch tube (labelled as‘‘Tube B’’) is inclined by 30� to the axis of the main body tube(labelled as ‘‘Tube A’’). Joining is performed by means of twoinclined, out-of-plane, plastic instability waves that closelymatch the intersection of the two tubes. Two different typesof mandrels are employed; (i) a conventional internal mandrelplaced inside the main body tube (‘‘Mandrel A’’) and (ii) a spe-cial purpose internal sectioned mandrel made of two differentparts (‘‘Mandrel B’’) placed inside the branch tube. The con-ventional mandrel avoids the development of unacceptableinward plastic flow during triggering and propagation of theplastic instability waves whereas the special purposed sec-tioned mandrel (allowing for the easy removal of mandrel inpractice) prevents the compression beads to plough into thebranch tube. The branch tube behaves as a sleeve during theentire joining process and its internal sectioned mandrel con-tributes decisively to ensure the overall success of the inclinedjoining process.

Figure 6b, allows understanding the role played by theinternal mandrel in preventing the development of plastic insta-bility waves exhibiting both inward and outward plastic flowduring the fixing of tubes to sheets in situations where the axisof the sheet is perpendicular or inclined to the axis of the mainbody tube (refer also to Figure 5b). The compression beadsresulting from these plastic instability waves exhibiting bothinward and outward plastic flow would lead to non-acceptablejoints and, therefore, justify the reason why the utilization ofinternal mandrels is mandatory in order to ensure the overallquality and tolerances that are required for the tube-sheetinclined connections.

5.2. End-to-end connections

Figure 7a shows a photograph of several tube specimensthat were connected by their ends using the new proposed tech-nology. The observation of the real and finite element predicted

Figure 5. Application of the new proposed technology for (a) connecting full-size and half-sectioned tubes and for (b) fixing tubes to sheets(metallic and polymeric) at different inclination angles.

6 L.M. Alves et al.: Manufacturing Rev. 2015, 2, 16

Figure 6. (a) Finite element model and predicted geometries at the middle and end of stroke with photograph of an inclined connection(a = 30�) between the two tubes. (b) Photograph and finite element predicted geometries disclosing the influence of the internal mandrel inthe development of sound asymmetric, in-plane, plastic instability waves for fixing tubes to sheets.

Figure 7. Modes of deformation associated to end-to-end joining of S460MC tubes by cold forming. (a) Experimental jointsand (b) experimental and finite element predicted geometries of the cross sections for specimens with different values of the slenderness ratiolgap/r0.

L.M. Alves et al.: Manufacturing Rev. 2015, 2, 16 7

cross sections in Figure 7b allows concluding that the leftmosttest sample (corresponding to lgap/r0 = 1.9) does not ensurelocking between the two tubes whereas the rightmost test sam-ple (corresponding to lgap/r0 = 4.4) presents a joint with twocompressions beads instead of one.

In case of the leftmost test sample, the absence of connec-tion is because the initial unsupported gap height lgap is not bigenough to allow compression beads to develop and lock witheach other by plastic instability. In case of the rightmost testsample, the formation of two compression beads instead ofone is due to the fact that high values of the initial unsupportedgap height lgap provide conditions for the development of mul-tiple compression beads that will interfere and be placed on topof each other, as they are formed in-between the upper andlower dies.

Neither the operative conditions corresponding to the left-most test sample nor those corresponding to the rightmost testsample are acceptable for connecting the two tubes by theirends. The process window is therefore restricted to values ofthe slenderness ratio lgap/r0 in the range between the twoabovementioned limits. However, it is worth noting that the pro-cess window must not be confused with the potential range ofapplicability because the limits on the slenderness ratio lgap/r0

only define the range of values of the unsupported heights of

the upper and lower tubes that need to be utilized for success-fully connecting any two tubes by their ends.

Figure 8 shows the application of the proposed technologyto the connection of two PVC tubes by their ends. Similarly towhat authors observed in metallic tubes, the end-to-end joiningof PVC tubes by cold forming is dependent upon the develop-ment and propagation of sound axisymmetric plastic waves atthe gap opening lgap between the upper and lower dies.

When the slenderness ratio lgap/r0 has a value smaller thanthe minimum threshold it is not possible to connect the PVCtubes by their ends because the initial gap opening lgap is notbig enough to allow plastic instability waves to develop andclamp the two tubes by mechanical locking (refer to theleftmost sample of Figure 8a, where lgap/r0 = 0.56. In contrast,when the slenderness ratio lgap/r0 has a value larger than themaximum threshold it is not possible to ensure a sound andreliable connection of the PVC tubes by their ends (refer tothe rightmost sample in Figure 8a where lgap/r0 = 3.75).Values of the slenderness ratio lgap/r0 in-between the leftmostand rightmost samples in Figure 8a are acceptable forconnecting the two PVC tubes by their ends by means of coldforming. The process window is therefore characterized byvalues of the slenderness ratio lgap/r0 in the range between1.8 and 3.2.

Figure 8. Modes of deformation associated to end-to-end joining of PVC tubes by cold forming. (a) Experimental joints and (b) experimentaland finite element predicted geometries of the cross sections for specimens with different values of the slenderness ratio lgap/r0.

8 L.M. Alves et al.: Manufacturing Rev. 2015, 2, 16

The observation of the different modes of deformation inthe end-to-end joining of PVC tubes by their ends adds anew physical phenomenon known as ‘‘stress whitening’’ thatdoes not occur in metallic tubes (Figure 8b). Stress whiteningconsists in the change of color with plastic deformation and isattributed to the occurrence of crazing in the regions of thePVC tubes subjected to high values of tensile stresses. Thisphenomenon is particularly important in PVC and Figure 8bincludes the finite element predicted distribution of crazingin accordance with the normalized version of Bucknall’s mod-ification of the stress-bias criterion due to Sternstein andOngchin [10].

6. Conclusions

The new proposed technologies for connecting tubes andtubes to sheets by triggering and controlling the propagationof plastic instability waves in thin-walled tubes offer severaladvantages as compared with conventional technologies basedon mechanical fixing with fasteners, welding or structuraladhesive bonding because they are:

d flexible solutions capable of handling small, medium orlarge batch sizes with different geometries and high lev-els of repeatability in production line;

d environmentally friendly solutions that allow savings inraw material and eliminate filler materials and shieldinggases;

d energy saving solutions that eliminate heat-coolingcycles as well as heat affected zones in the regions ofthe tubes and sheet panels that are joined together;

d value added solutions that are capable of connectingtubes and fixing tubes to sheet panels made of dissimilarmaterials;

d cost-efficient solutions that require low amount of capitalinvestment because they can be designed to operate withexisting machine-tools.

Because the new proposed technologies can be success-fully employed in fixture conditions that are difficult and costly

to achieve by means of conventional technologies they can alsobe utilized to foster innovative ideas in product development.

Acknowledgements. The authors would like to acknowledge thesupport provided by Fundação para a Ciência e a Tecnologia ofPortugal under LAETA – UID/EMS/50022/2013 and by MCG –Mind for Metal, Carregado, Portugal.

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Cite this article as: Alves LM, Silva CMA & Martins PAF: Innovative cold joining technologies based on tube forming. ManufacturingRev. 2015, 2, 16.

L.M. Alves et al.: Manufacturing Rev. 2015, 2, 16 9